Thermionic valve utilizing secondary electron emission amplification



ATHERTON THERMIONIC VALVE UTILIZING SE 2,553,997 CONDARY May 22, 1951 A.H.

ELECTRON EMISSION AMPLIFICATION Filed Jan. 19, 1949 FIG.

i atented May 22, 195 1 ARY ELECTRON TION EMIS SION AMPLIFICA AlbertHorace Atherton, London, England, as- Signor to Electric & MusicalIndustries Limited, Hayes, England, a company of Great BritainApplication January 19, 1949, Serial No; 71,668 In Great Britain January24, 1948 16 Claims. (01. est-27.5)

This invention relates to thermionic valves adapted to utilize secondaryelectron emission for I obtaining amplification.

Various proposals have been made heretofore for constructing amplifyingvalves comprising a thermionic cathode, a control electrode to whichsignals to be amplified can be applied so as to control the emission ofprimary electrons from said thermionic cathode, a secondaryelectronemitting electrode (referred to hereinafter and in the claims asa secondary cathode) on which the primary electrons are caused toimpinge so that a greater number of secondary electrons are emitted thanthere are impinging primary electrons, and a collecting electrode forcollecting the secondary electrons so that-the amplified signals can beobtained from a'ioad impedance connected to the collecting electrode. 7

can pass to bombard said secondary cathode to.

cause the emission therefrom of secondary eleca trons, a controlelectrode capable of controlling the emission of electrons from saidthermionic cathode, and a collecting electrode capable of collectingsecondary electrons from said secondary cathode, said control electrodeand said secondary electrode being disposed in succession between saidthermionic cathode-and said secondary cathode and being pervious toelectrons flowing in said path, and electron beam-forming means, thearrangement beingsuch that in operation of the valve the electronsemitted from said thermionic cathode are formed by said beamformingmeans into a beam of electrons which is caused to flow along said pathto said secondary cathode without the beam being deflected.

According to a preferred form of this aspect of the present inventionthere is provided a thermionic valve adapted to utilizesecondaryelectron emission for obtaining amplification, comprising athermionic cathode and a secondary cathode between which there isprovided a rectilinear space current path along which electrons emittedfrom said'thermionic cathode can pass to bombard said secondary cathodeto cause the emission therefrom of secondary electrons, a controlelectrode capable of controlling the emission of electrons from saidthermionic cathode, an accelerating electrode capable of acceleratingthe electrons emitted from said thermionic cathode, and a collectingelectrode capable of collecting secondary electrons from said secondarycathode, said control electrode, accelerating electrode and" collectingelectrode being disposed in sucsession between said thermionic cathodeand said secondary cathode and being pervious to electrons flowing insaid path, and electron beam-forming means disposed between saidaccelerating electrode and said collecting electrode, the arrangementbeing such that in operation of the valve the electrons emitted fromsaid thermionic cathode are formed by said beam-forming means into abeam of electrons which is caused to flow along said path to saidsecondary cathode without the beam being deflected.

According to another aspect of the present invention there is provided athermionic valve adapted to utilize secondary electron emission forobtaining amplification, comprising a thermionic cathode and a secondarycathode between which there is provided a rectilinear space current pathalong which electrons emitted from said thermionic cathode can pass tobombard said secondary cathode to cause the emission therefrom ofsecondary electrons, a control electrode capable of controlling theemission of electrons from said thermionic cathode, and a collectingelectrode capable of collecting secondary electrons from said secondarycathode, said secondary cathode having a coating of secondary electronemitting material therefrom which comprises refractory oxides, 20 to percent of the total weight of said refractory oxides being alkaline earthoxide.

During the manufacture of valves which are adapted to utilize secondaryelectron emission it is desirable to activate both the primary andsecondary electron-emitting electrodes by heating them in vacuo to arelatively high temperature, but if as in the example described in thepreceding paragraph there is a rectilinear path between the thermioniccathode and a secondary cathode there is a risk of one of said cathodesbeing contaminated by the deposition of matter driven off from the othercathode during activation thereof.

Therefore the object of another feature of the invention is to providean improved method of activating the electron emitting electrodes 'ofthermionic valves adapted to utilize secondary electron emission, with aview to reducing the risk referred to.

According to said feature of the present inven tion there is provided amethod of activating the electrodes of a thermionic valve adapted toutilize secondary electron emission for obtaining amplification andwhich comprises a thermionic cathode and a secondary cathode, whereinsaid cathodes are heated simultaneously tofsucli temperatures that thecontamination of one of said cathodes by matter driven oiiv .by heatingsaid are extensions l3 on opposite sides of the path anamplifying valveit'may for example be emother cathode is effectively avoidedandthe heating of said cathodes is simultaneously discontinued.

The heating of the secondarycathode may be effected wholly or in part bybombardment with electrons liberated from the thermioniccathode in whichcase of course the secondary cathode will attain its activatingtemperature later than the thermionic cathode, but this is immaterialprovided the heating of the two electrodes is discontinuedsimultaneouslyl The secondary cathodemay alternatively or additionallybe heated by means of a separate heater.

In order that the said invention may be clearly understood and readilycarried into effect, the same will now be more fully described withreference to the accompanying drawing, wherein- Figure 1 illustratesdiagrammatically in transverse section a thermionic valve in accordancewith one example of the present invention, and

Figure 2 is a symbolic diagram illustrating a circuit arrangementembodying the valve illustrated in Figure 1.

Referring to the drawingfthe valve comprises a tubular thermionic cathodof rectangular section and enclosing a heater 2,a control electrode 3,an accelerating electrode 4, a suppressor electrode 5, two electronbeam-forming plates two secondary cathodes i and two collectingelectrodes or anodes 3. The electrodes 3, i and 5 comprise wire woundgrids surrounding the cathode I and supported in known manner'on rods S,while the cathode has two substantially plane surfaces iii coated withthermionic elec tron-emitting material. The surfaces It face thesecondary electron-emitting surfaces H of the secondary cathodes I, thelatter surfaces being also plane and coated with a suitable secondaryelectron-emitting material comprising, as described in the United StatesPatent application Ser. No. 711,860, refractory oxides per cent to 50per cent of the total weight of which is alkaline earth oxide, theexpression alkaline earth oxide being intended to apply only to theoxides of calcium, strontium and barium. Preferably the coating materialcomprises magnesium oxide and barium oxide, the latter oxideconstituting from 20 per cent to 50 per cent of the total weight of thelatter two oxides, as described in the aforesaid co-pending UnitedStates application, and preferably also the coating is at least 10inches thick. The collecting electrodes 8 comprise planar wir gridsarranged in front of the surfaces ll of the secondary cathodes l andparallel thereto, the collecting electrodes 8 being supported on rodsl2. As can be seen in the drawing there is a rectilinear space currentpath between each surface lll and the corresponding surface H and suchelectrodes as are disposed in said path are pervious to electrons, sothat effectively said path is uninterrupted. The beamforming plates 5are of curved section as shown having inturned extensions 13, such thatthere bodied in a circuit such as illustrated in Figure 2, and, forconvenience, only one of the secondary cathodes l and one of thecollecting electrodes 8 is'indicated in'the valve in Figure 2. As showntherein the;suppressor electrode the plates 5 and hence also the,extensions l3 are maintained at the potential of the cathode l, which isearthed in' this example. The control electrode 3 is connected viaa leakresistance 96 to a source of bias potential of about 1.5 to 2 voltsnegative indicated diagrammatically by the arrow ii, while the signalsto be amplified can be applied to the control electrode via the couplingcondenser I53. The accelerating electrode 4 is connected to a source isof positive potential of say 250 volts positive, the secondary cathodes'i are connected to a source it of positive potential, also of 250 voltsfor example, while the collecting electrodes 8 are connected to a source2! of positive potential of say 350' volts positive via a loadresistance 22. The load'resistance 22 may alternatively be connected inthe external lead to the secondary cathode, but it will be appreciatedthat in either case the load impedance is connected in theanode-to-secondary cathode circuit of the valve. Where necessary theelectrodes of the valve will be decoupled in known manner but in theinterests of clearness such decoupling is not illustrated in thedrawing. In operation of the valve the plates 6 and with them theextensions l3 serve,

in conjunction with the other electrodes, to concentrate the electronsemitted from each surface 96 of the thermionic cathode and so form twobeams, indicated at 23 in Figure l, which without being deflected flowalong the rectilinear paths between the surfaces Hi and the surfaces i iand bombard the latter surfaces, whereby more than one secondary,electron is emitted by each bombarding electron. The secondary electronsare collected by the collecting electrodes 8. The number of electrons inthe beams 23 is controlled by the signals applied to thecontrolelectrode 3, and an amplified output of said signals is set up acrossthe resistance 22 and can be fed by means of a condenser 24 to asubsequent stage. The collecting electrodes 8 are arranged close to theemitting surfaces l l of the secondary cathodes i, at say between 0.5and 1 mm. from said surfaces, so as. to provide a high field intensityat said surfaces ll since it is found that with a greater separationbetween the collecting electrodes 8 and. the emitting surfaces of thesecondary cathodes i a higher positive potential is required at thecollecting electrodes iiin order to saturate the secondary electroncurrent in the device.

During the manufacture of the valve, after the electrodes have beenmounted in the envelope i5 and the envelope evacuated, activation of thethermionic cathode I and the secondary cathodes l is effected by heatingin such manner that finally, at least, the electrodes! and l aremaintained simultaneously at about 1100 C. for about 2 minutes.Bombardment with primary electrons emitted from the thermionic cathodethe electrodes 1 may ifdesired be provided with separate heaters, inwhich case, as shown in the drawing, the electrodes I are preferablytubular and of rectangular section, with heaters 25 provided in theinterior of the electrodes. It is also possible to utilizeeddy currentheating for activating the secondary cathodes 1.

The construction illustrated in the drawing enables the size of thevalve to be considerably reduced and its manufacture simplifiedincomparison, for example, with the valve illustrated in United Statespatent application Ser. No. 695,531, while at the same time similaroperating characteristics can be obtained. For example a slope of 14miilliamperes per volt has been obtained with valves of the kindillustrated in the drawing, the anode current being about 15milliamperes and the valve being dimensioned to give a current in theelectron beams impinging on the electrodes '7 of about 5 milliamperesper square centimeter of the total emitting surface.

Modifications may of course be made of the construction illustrated; forexample the thermionic cathode I may have other than planaremittingsurfaces, for instance the cathode may be of circular section, and thebeam forming plates 6 may be replaced by conductive rods, while in somecases the suppressor electrode 5 may be dispensed with. Moreover, thesecondary electron-emitting surfaces ll of the secondary cathodes may beother than planar, for example they may be convex as seen from thethermionic cathode.

What I claim is:

1. In an amplifying circuit, a thermionic valve adapted to utilizesecondary electron emission for obtaining amplification, comprising athermionic cathode and a secondary cathode between which there isprovided a rectilinear space current path along which electrons emittedfrom said thermionic cathode pass to bombard said secondary cathodetocause the emission therefrom of secondary electrons, a controlelectrode for controlling the emission of electrons from said thermioniccathode, a collecting electrode for collecting secondary electrons fromsaid secondary cathode, and electron beam-forming means for forming theelectrons emitted from said thermionic cathode into a beam of electroncaused to flow without substantial deflection along said path to impingeon said secondary cathode with a density of the order of 5 milliamperesper square cm. of emitting surface of said secondary cathode, saidsecondary cathode being provided with an electron-emissiive coatingwhich is at least 10- inches thick and consisting of refractory oxidematerial.

2. A thermionic valve adapted to utilize secondary electron emissive forobtaining amplification, comprising a thermionic cathode, a secondarycathode facing the electron emissive surface of said thermionic cathode,a control electrode for controlling the emission of electrons from saidthermionic cathode, a collecting electrode for collecting secondaryelectrons from said secondary cathode, said control electrode andcollecting electrode being disposed in succession between saidthermionic cathode and said secondary 'cathode and being pervious toelectrons to provide a substantially rectilinear space current pathbetween said cathodes, and said secondary cathode comprising aconductive support provided with secondary electron-emissive coating atleast 10* inches thick and consisting. of a mixture of magnesium oxideand barium 6.v oxide, the barium oxide constituting percent of saidmixture.

3. In an amplifying circuit, a thermionic valve adapted to utilizesecondary emission for obtaining amplification, comprising a thermioniccathode and a secondary cathode, between which there is provided arectilinear space current path along which electrons emitted from saidther mionic cathode pass to bombard said secondary from 20 to cathode tocause the emission therefrom of secpervious to electrons flowing in saidpath, and. electron beam forming means disposed between. saidaccelerating electrode and said collecting electrode to form primaryelectrons emitted from: said thermionic cathode into a beam ofelectrons: caused to flow without substantial deflection along said pathand to impinge on said secondary cathode with a density of about 5milliamperes per square cm. of emitting surface of said secondarycathode.

4. A thermionic electron discharge valve constructed to utilizesecondary-electron emission for obtaining amplification, said valveincluding a thermionic cathode having an electron emissive surface, anda secondary cathode comprising a conductive support and asecondary-electron emissive coating on a surface of said support, saidcoating comprising refractory oxides of which from 20 to 50 per cent isalkaline earth oxide, and. said support being disposed with said secondsurface in a direct electron path from said first surface.

5. A valve according to claim 4, comprising beam forming means disposedbetween said thermionic cathode and said secondary cathode for formingelectrons emitted from said first surface into a beam of electronsdirected to said second surface.

6. A thermionic electron discharge valve constructed to utilizesecondary-electron emission for obtaining amplification, and said valveincluding a thermionic cathode having an electron emissive surface, anda secondary cathode comprising a conductive support and asecondaryelectron emissive coating at least 10 inches thick on a surfaceof said support, said coating comprising refractory oxides of which from20 to 50 per cent is alkaline earth oxide, and said.

support being disposed with said second surface in a direct electronpath from said first surface.

7. A thermionic electron discharge valve constructed to utilizesecondary-electron emission for obtaining amplification, said valveincluding a thermionic cathode having an electron emissive surface, anda secondary cathode comprising a conductive support and asecondaryelectron emissive coating on a surface of said support, saidcoating consisting of magnesium oxide and barium oxide, said bariumoxide constituting from 20 to 50 per cent by weight of the coating, andsaid support being disposed with said second surface in a directelectron path from said first surface.

8. An amplifying circuit embodying a ther mionic electron dischargevalve constructed to utilize secondary-electron emission for obtainingamplification, said valve comprising at least a thermionic cathodehaving an electron emissive surface, a secondary cathode having asecondar-yelectron emissive surface facing said first surface, a controlelectrode for controlling the emission of electronsfrom said firstsurface in response to signals applied to said circuit, an acceleratingelectrode, a collecting electrode, said control electrode, acceleratingelectrode and collecting electrode beingarranged in succession betweensaid thermionic cathode and said secondary cathode and being pervious toelectrons to provide a rectilinear electron path between.

said cathodes, beam forming members for formingelectrons emitted fromsaid first surface into a beam ;of electronsdirected along said path tosaid second surface, said members being disposed on opposite sides ofsaid'paths between said acceleratingelectrode and collecting electrodeand next adjacent said collecting electrode, and mean maintainingpositive potentials with reference to said thermionic cathode at saidaccelerating electrode, secondary cathode and collecting electrode, withthe last electrode most positive, whereby secondary electrons from saidsecondary cathode are collected mainly by said collecting electrode.

9. A circuit according to claim 8, said secondary cathode comprising aconductive support having a surface facing said first surface, and acoat: ing of secondary electron emissive material on said surface of thesupport, said coating comprising refractory oxides of which from 20 to50 per cent is alkaline earth oxide.

10. An amplifying circuit embodying a thermionic electron dischargevalve constructed to utilize secondary-electron emission for obtainingamplification, said valve comprising at leasta thermionic cathode havingan electron emissive surface, a secondary cathode having a planarsecondary-electron emissive surface facing said first surface, a controlelectrode for controlling the emission of electrons from said firstsurf-ace inresponse to signals applied to said circuit, an

accelerating electrode, a planar apertured collecting electrode parallelto said second surface, said control electrode, accelerating electrodeand collecting electrode being arranged in succession between saidthermionic cathode and said secondary cathode and being pervicus toelectrons to provide a rectilinear electron path between said surfaces,beam forming means for forming electrons emitted from said first surfaceinto abearn of electrons directed along said path to said secondsurface, said means being disposed be, tween said accelerating electrodeand collecting electrode and next adjacent to said collecting electrode,and means maintaining positive potentials with reference to saidthermionic cathode at said accelerating electrode, secondary cathodeandcollecting electrode with the last electrode most positive, wherebysecondary electrons from said secondary cathode are collected mainly bysaid collecting electrode.

11. A circuit according to claim 10, said secondary cathode comprising aconductive support having a planar surface facing and parallel with saidfirst surface, and a secondary electron emissive coating on said supportsurface, said coating consisting of magnesium oxide and barium oxide ofwhich from 20 to 50 per cent by weight is barium oxide.

12. A circuit according to claim 11, said coating being at least 10*inches thick.

13. amplifying circuit embodying a thermionic electron discharge valveconstructed to utilize secondary-electron emission for obtainingamplification, said valve'comprising at least a thermionic cathodehaving an electron emissive surface, a secondary cathode having a planarsecondary-electron emissive surface facing said first surface, acontrolelectrode for controlling the emissionof electrons from said firstsurface in responseto signals applied to said circuit, an acceleratingelectrode, a planar apertured collecting electrode parallel to'saidsecond surface, said control electrode, accelerating electrode andcollectingelectrode being arranged in succession between said thermioniccathode and said'secondary cathode and being pervious to electrons toprovidea rectilinearelectron path between said surfaces, and beamforming means disposed between said accelerating electrode andcollecting electrode and-next adjacent to said collecting electrode forforming electrons emitted from said first surface-into a beam-ofelectrons fiowing without substantial deflection along said path toimpinge on said second surface with a density of about 5mil-liamperesper square centimeter of said second surface.

14. An amplifying circuit embodying a thermionic electron dischargevalve constructed to utilize secondary-electron emission for obtainingamplification, said. valve comprising at least a thermionic cathodehaving an .electron emissive surface, a secondary cathode having asecondaryelectron emissive surface facing said first surface, a controlelectrode for controlling the emission of electrons from said firstsurface in response to signals applied to said circuit, an acceleratingelectrode, a collecting electrode, said control electrode, acceleratingelectrode and collecting electrode being arranged in succession betweensaid thermionic cathode and said secondary cathode and being pervious toelectrons to provide a retilinear electron path between said cathodes,beam forming members for forming electrons emitted.

from said first surface into a beam of electrons directed along saidpath to said second surface, said members being disposed on oppositesides of said paths between said accelerating electrode and collectingelectrode and next adjacent said collecting electrode, means formaintaining said beam forming members at the same potential as that ofsaid thermionic cathode, and means maintaining positive potentials withreference to said thermionic cathode at said accelerating electrode,secondary cathode and collecting electrode with the last electrode mostpositive.

15. A circuit according to claim 14 comprising a suppressor electrode insaid valve disposed between said accelerating electrode and said beamforming members, and means for maintaining said suppressor electrode atthe same potential as that of said beam forming members.

16. A thermionic electron discharge valve constructed to utilizesecondary-electron emission for obtaining amplification, said valveincluding a thermionic cathode having an electron emissive surface, asecondary cathode comprising a conductive support andasecondary-electron emissive coating on a surface of said support, saidcoating comprising refractory oxides of which from 20 to ,50 per cent isalkaline earth oxide and said support being disposed with said secondsurface in a direct electron path from said first sur face, and saidthermionic cathode and said secondary cathode being arranged forsimultaneous heatingflto such'temperatures that contamination of'one ofsaid cathodes by matter driven off 9 by heating the other of saidcathodes is effectively avoided, the heating of said cathodes beingsimultaneously discontinued.

ALBERT HORACE ATHERTON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,146,607 Van Overbeek Feb. 7,1939 2,151,783 Lopp et a1 Mar. 28, 1939 2,159,774 Veenemans et al. May23, 1939 2,164,892 Banks July 4, 1939 2,167,097 Van Overbeek et a1. July25, 1939 OTHER REFERENCES Secondary Emission, (Part I) by L. R. Koller,in April 1948, General Electric Review (pages 33 through 40).

"Secondary Emission, (Part II), by L. R. Koller, in June 1948, GeneralElectric Review (pages 50 through 52).

